Structure and mechanical properties of oxygen doped diamond-like carbon thin films

    Structure and mechanical properties of oxygen doped diamond-like carbon thin films Pouria Safaie, Akbar Eshaghi, Saeed Reza Bakhshi PII: DOI: Reference:

S0925-9635(16)30400-9 doi:10.1016/j.diamond.2016.10.008 DIAMAT 6732

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Diamond & Related Materials

Received date: Revised date: Accepted date:

1 August 2016 13 October 2016 13 October 2016

Please cite this article as: Pouria Safaie, Akbar Eshaghi, Saeed Reza Bakhshi, Structure and mechanical properties of oxygen doped diamond-like carbon thin films, Diamond & Related Materials (2016), doi:10.1016/j.diamond.2016.10.008

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ACCEPTED MANUSCRIPT Structure and mechanical properties of oxygen doped diamond-like carbon thin films

Pouria Safaie, Akbar Eshaghi*, Saeed Reza Bakhshi

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Faculty of Materials Science and Engineering, Malek Ashtar University of Technology, Shahin shahr, Esfahan, Iran.

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Abstract

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In this study, structure and mechanical properties of doped diamond-like carbon (DLC) films with oxygen were investigated. A mixture of methane (CH4), argon (Ar) and oxygen (O2) was used as feeding gas, and the RF-PECVD technique was used as a deposition method. The thin films were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and a combination of elastic recoil detection analysis and Rutherford backscattering (ERDA-RBS). Nano-indentation tests were performed to measure hardness. Also, the residual stress of the films was calculated by Stoney equation. The XPS and ERDA-RBS results indicated that by increasing the oxygen in the feeding gas up to 5.6 vol. %, the incorporation of oxygen into the films’ structure was increased. The ratio of sp2 to sp3 sites was changed by the variation of oxygen content in the film structure. The sp2/sp3 ratios are 0.43 and 1.04 for un-doped and doped DLC films with 5.6 vol. % oxygen in the feeding gas, respectively. The Raman spectroscopy (RS) results showed that by increasing the oxygen content in doped DLC films, the amount of sp2 C=C aromatic bonds was raised and the hydrogen content reduced in the structure. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the decrease of hydrogen content and the increase the ratio of C=C aromatic to olefinic bonds. Hardness and residual stress of the films were raised by increasing the oxygen content within the films’ structure. The maximum hardness (19.6 GPa) and residual stress (0.29 GPa) were obtained for doped DLC films, which had the maximum content of oxygen in structure, while the minimum hardness (7.1 GPa) and residual stress (0.16 GPa) were obtained for un-doped DLC films. The increase of sp3 C-C bonds between clusters and the decrease of the hydrogen content, with a simultaneous increase of oxygen in the films’ structure is the reason for increase of hardness and residual stress. Keywords: Diamond-like carbon, RF-PECVD, oxygen, Hardness, Residual stress

1. Introduction

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Corresponding author. Fax: +98 3125228530. E-mail address: [email protected]

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Diamond-like carbon (DLC) is a metastable form of amorphous carbon that has unique properties, such as high mechanical hardness, chemical inertness and optical transparency. Due to these unique properties, DLC films can be appropriate for application as protective coatings [1]. Hydrogenated amorphous carbon (a-C:H) films with intermediate H content (20-40 at. %) are one type of DLC films that are usually deposited by PECVD methods with precursor hydrocarbon gases [1,2]. Radio frequency plasma enhanced CVD (or RF-PECVD) is the most common deposition method of diamond-like a-C:H (DLC) due to a simple deposition system, the ability to accurately control the chemical composition, the inherently uniform plasma, the low temperature process (room temperature up to 250 °C) and the relatively low residual stress in deposited films [3,4]. The use of DLC films is limited due to intrinsic compressive residual stress, which may cause poor film adhesion to the substrate [5]. A film with the thickness h will delaminate when the elastic energy E per unit volume due to stress σ exceeds the surface fracture energy γ. To achieve good adhesion the thickness h has to be less than 4γE/σ2 [1]. Doping or alloying the DLC films with light elements (B, Si, O, N, and F) and metals is an effective way to modify their properties such as internal stress and hardness [6]. The incorporation of dopant elements into the DLC film’s structure causes to change the content of hydrogen and the ratio of sp2/sp3 sites [7-10]. Thus, the properties of the DLC films are changed. There are several researches that studied doped DLC with nitrogen [7,11], silicon [8,12], boron [9,13] and fluorine [10,14], but few researches were done about oxygen doped DLC [15-18]. On the other hand, by changing the deposition method and deposition parameters such as kind of precursor hydrocarbon gas, chamber pressure and power (or bias voltage), the structure of DLC films and consequently their properties are varied [19]. In this study, we deposited oxygen doped DLC films by using a CH4-Ar-O2 gas mixture. We investigated the effect of oxygen doping on the structure and mechanical properties of DLC films. In contrast to few previous researches, we observed the different trend in change of mechanical properties.

2. Experimental

The oxygen doped DLC films were deposited using conventional RF-PECVD (13.56 MHz, Plasma Fan Co, Iran) method. A mixture of methane (CH4), argon (Ar) and oxygen (O2) was used as a feeding gas. The ratio between precursor gases, CH4 : Ar : O2, was 9 sccm : 1 sccm : X sccm (X=0.0, 0.2, 0.4, 0.6, 0.8, 1.0). Polished silicon and thin borosilicate glass D263T (~160 µm thickness) were selected as substrate materials. More detail about the deposition process and the preparation of substrates before deposition has been reported in previous work [20]. X-ray photoelectron spectroscopy (XPS) was used to analyze carbon bonding configurations and to estimate oxygen and carbon concentrations. The films were irradiated in high vacuum by a monochromatic Al kα X-ray beam (hν=1486.6 eV). In order to avoid hybridization transformation of carbon pre-clean sputtering treatment was not performed [21]. A combination of elastic recoil detection analysis and Rutherford backscattering (ERDA-RBS) was used to determine the chemical composition. The Raman spectroscopy (RS) was used to investigate the bonding structure of the films. The wavelength of exciting beam was 532 nm and was created by Nd:YLF laser. Dispersed Raman spectra were collect with 4 cm-1 resolution. Attenuated total 2

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reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to characterize the bonding in the films. Nano-indentation test were performed to measure mechanical properties of the films such as hardness [22]. In order to have accurate data, the indenter penetrates into 10 % of the film thickness [23]. The curvature of the thin borosilicate glass substrate was measure using the optical instrument, before and after the deposition, and residual stress of the films were calculated by Stoney equation [24]. According to Jiang [25], the Poisson’s ratio is about 0.3 for PECVD DLC films, thus this value was used to calculate the residual stress.

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3. Results and discussion 3.1. XPS and ERDA-RBS analysis

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X-ray photoelectron spectroscopy was used for two purposes. The main purpose was the determination of weight ratio of sp3 and sp2 sites in the film structure. The secondary purpose was to estimate oxygen and carbon concentrations in the film structure. In order to determine chemical composition exactly, a combination of elastic recoil detection analysis and Rutherford backscattering (ERDA-RBS) was used for un-doped and doped DLC films with 0.6 sccm oxygen flow rate. Casa XPS software was used to analyze the XPS spectra and to extract quantitative data from them. The background was subtracted by Shirley method and then the chemical composition was determined by measuring the peak area and considering the sensitive peak factors (2.93 for carbon and 1 for oxygen) [26]. The Casa XPS software was also used to deconvolute the C1s peak into three sub-peaks, which are attributed to sp3 (C-C or C-H), sp2 (C=C) and CO bonds in DLC film structure [27]. The oxygen peak was selected as a reference peak to calibrate the XPS spectra (O1s was set at 532 eV). The XPS spectra of the un-doped and doped DLC films with 0.6 sccm oxygen flow rate in the range of 0 to 1200 eV are shown in Fig. 1. Two major peaks were observed in all films’ spectra. One peak was around 284 eV that is related to C1s and another peak was around 532 eV that is related to O1s. Moreover, a relatively weak peak was observed around 983 eV that is related to OKLL Auger electron. Due to chemisorbed oxygen pieces, after deposition and exposure of the films to the air, the O1s peak exists in XPS spectra of the un-doped DLC film [27,28]. Atomic percentage of carbon and oxygen, elements that were extracted from XPS spectra, are summarized in Table 1. Since the H content cannot be measured by XPS technique, the sum of C and O atomic concentration is 100 at. %. In order to detect the oxygen concentration in the doped DLC films, the oxygen content of the un-doped DLC film was subtracted from the oxygen content of the doped DLC films. The oxygen concentration of the doped DLC films with 0.2, 0.4, 0.6, 0.8 and 1.0 sccm flow rate are 1.59, 2.42, 6.42, 1.88, and 2.57 at. %, respectively. It is observable that by increasing the oxygen flow rate up to 0.6 sccm in the feeding gas, the incorporation of oxygen into the film structure increased. The ERDA-RBS results that are shown in Table 2 confirmed this claim. At 0.8 and 1.0 sccm oxygen flow rate, the oxygen content of the films was less than in the doped film with 0.6 sccm oxygen flow rate. The doped film with 0.6 sccm oxygen flow rate had the maximum oxygen content.

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3.2. Raman spectroscopy

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The detailed C1s spectra of all the films, that were recorded in the binding energy range 280-290 eV with 0.03 eV resolution, are shown in Fig. 2. Three sub-peaks are also shown in Fig. 2 that are related to sp3, sp2 and CO bonds. The position, full width at half maximum (FWHM) and area of the peaks are summarized in the Table 1. In order to investigate the C1s peak position shift accurately, the position of C1s, sp3, sp2 and CO peaks were calibrated again (sp3 peak was set at 284.3 eV) [27]. Carbon is a homo-polar atom, so the shift of its binding energy is small. The difference between the binding energy of the C1s peak of graphite and diamond is 0.7 eV [29]. This is due to the slightly shorter bond length of sp2 bonds than sp3 bonds [30]. Thus, sp2 sites show a slightly deeper potential than sp3 sites. The peak area is proportional to the bond content in the film. The CO bond content increased simultaneously with an increase of the oxygen content in the films’ structure. The maximum of CO bonds was found in doped films deposited with 0.6 sccm oxygen flow rate. It can be seen that by increasing the oxygen incorporation in the film’s structure, the sp2/sp3 ratio was raised. The un-doped DLC film has a minimum value of the sp2/sp3 ratio of 0.43, and the doped DLC film with a 0.6 sccm flow rate of oxygen has maximum of 1.04. The C1s pack position of the oxygen doped DLC films were located at lower binding energy than the C1s peak of the un-doped DLC film. The C1s packs of the oxygen doped DLC films were wider than those of the un-doped DLC films. By increasing the oxygen content in the film’s structure, CO and sp2 bond content increased. Increasing of sp2 bonds causes to shift the C1s position to lower binding energies. Increasing CO bonds causes to widen the C1s peak. The addition of oxygen to the films’ structure encourages the formation of sp2 (C=C) hybridization and discourages the formation of sp3 (C-C or C-H) hybridization.

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The Raman spectra of all films were collected in the range from 900 to 1900 cm-1, as shown in Fig. 3. A relatively broad peak in this range appeared that is a feature of the DLC films [31]. This peak is created by the overlapping of two peaks, G (graphite) and D (disorder) peaks. The G peak is due to stretching vibration of any pair of sp2 sites, whether in C=C chains or in aromatic rings, but the D peak is due to breathing mode of those sp2 sites only in rings, not in chains [1]. The Raman spectra of the films were deconvoluted into G and D peaks with Gaussian line shapes. The G peak position, the full width at half maximum (FWHM) of the G peak and the ratio of the D peak intensity to G peak intensity (ID/IG) are parameters, which depend on the film’s structure [31]. These parameters were extracted from Raman films’ spectra and are summarized in Table 3. In the DLC films, the variation of G peak position and ID/IG ratio are in the same direction, and are in the opposite direction of the G peak width [32]. This trend is observable here. It can be seen that by increasing the incorporation of oxygen in the film’s structure, the G peak position shifted to higher value wave numbers. When the ID/IG ratio increases, the number of rings per cluster increases and the fraction of chain groups decreased [31]. Since the olefinic sites (chain-like) have more carbon-hydrogen bonds than aromatic sites (rings), it is expected that the hydrogen content of the DLC films were decreased by increasing the incorporation of oxygen in the film’s structure. The change in the films’ hydrogen content was investigated in part 3.3. Furthermore, the ERDA-RBS results showed the decrease in hydrogen content by increasing the incorporation of oxygen in the film’s structure. The shift of the G peak position from around 1518 cm-1 for un-doped DLC films to around 1576 cm-1 for 4

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doped DLC films with 0.6 sccm oxygen flow rate confirms the change of structure from mainly olefinic to mainly aromatic [31]. Although Raman spectroscopy dose not measured the sp3 fraction directly, the variation of sp3 fraction can be estimated by this method [1]. The increase of the ID/IG ratio and the shift of the G peak to around 1580 cm-1 indicated that sp2 sites increased and sp3 sites decreased [31]. The variation of the ID/IG ratio as a function of the sp2/sp3 ratio, which was extracted from XPS spectra, is shown in Fig. 4. These two ratio values were raised when the incorporation of oxygen in the films’ structure was increased. The approximate linear relation can be seen between these two ratio values.

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3.3. ATR-FTIR spectroscopy

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The characterization of the bonding in the DLC films by IR spectroscopy is the common method. In several literature reports, the vibrational frequencies wavenumber of possible bonds that exist in the DLC films were studied [33-35]. The carbon-hydrogen bond stretching vibration frequencies (2800-3300 cm-1) are higher than carbon-carbon bond stretching vibration frequencies (around 1600 cm-1) [33]. The carbon-hydrogen stretching modes breakdown into three regions, the sp1 ≡C-H modes center on 3300 cm-1, the sp2 =C-Hn mode lie from 2975 to 3085 cm-1 and sp3 -C-Hn mode lie from 2850 to 2955 cm-1 [33]. The normalized absorption spectra of the un-doped DLC film and doped DLC film with 0.6 sccm oxygen flow rate were derived from ATR-FTIR spectra. Two principle absorption regions were observable in the films’ absorption spectra that were located in the ranges from 2800 to 3000 cm-1 and 1500 to 1800 cm-1, and are shown in Fig. 5 and 6, respectively. The stronger absorption region (2800 to 3000 cm-1) was deconvoluted into four Gaussian line shape peaks. The centers of these sub-peaks are located at 2885 (sp3 CH3) cm-1, 2855 cm-1 (sp3 CH2), 2920 cm-1 (sp3 CH and sp3 CH2) and 2960 cm-1 (sp3 CH3). Another absorption region (1500 to 1800 cm-1) was also deconvoluted into four Gaussian line shape peaks. The centers of these peaks are located at 1550 cm-1 (sp2 aromatic C=C), 1580 cm-1 (sp2 aromatic C=C) 1650 cm-1 (sp2 olefinic C=C) and 1732 cm-1 (C=O bond). The positions and peak areas of all sub-peaks, for un-doped and doped DLC film with 0.6 sccm oxygen flow rate, are summarized in the Table 3. Since the peak area directly depends on the bond content of the films’ structure, the peak area is a criterion for measuring the bond content. It can be seen that the sp3 C-H content decreased by increasing the incorporation of oxygen in the film’s structure. The sp3 C-H content of doped DLC films with 0.6 sccm oxygen flow rate is about 0.82 times of that of the un-doped DLC film (A’1+A’2+A’3+A’4/A1+A2+A3+A4 =0.82). On the other hand, it can be seen that the sp2 C=C content (olefinic and aromatic) was raised by increasing the incorporation of oxygen in the films’ structure. The sp2 content of the doped DLC film with 0.6 sccm oxygen flow rate is about 1.28 times of that of un-doped DLC films (A’5+A’6+A’7/A5+A6+A7=1.28), however the increase of aromatic bonds is higher than the increase of olefinic bonds (A5+A6/A7=0.21 for un-doped DLC film and A’5+A’6/A’7=0.27 for doped DLC film with 0.6 sccm oxygen flow rate). No peaks were found in the range from 2975 to 3085 cm-1. This means that the content of sp2 C-H bonds in the films is negligible. The existence of an absorption peak around 1732 cm-1 (C=O) in the un-doped DLC is related to chemisorbed oxygen pieces. Due to incorporation of oxygen in the film’s structure and the formation of carbon-oxygen bonds into DLC network, the C=O peak area of the doped DLC film

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ACCEPTED MANUSCRIPT is more than that of the un-doped DLC film (A8=0.070 for un-doped DLC film and A’8=0.102 for doped DLC film with 0.6 sccm oxygen flow rate). 3.4. Mechanical properties (hardness and residual stress)

4. Conclusion

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Hardness and residual stress of the un-doped and doped films with oxygen are shown in Fig. 7. The variation of these mechanical properties of all films are in the same direction. This trend is common in the DLC film that hardness, elastic modules and residual stress simultaneously are varied in the same direction [1,36]. The doped DLC films with 0.6 sccm oxygen flow rate have maximum residual stress (0.29 GPa) and hardness (19.6 GPa) while the un-doped DLC films have minimum residual stress (0.16 GPa) and hardness (7.1 GPa). The mechanical properties of the DLC films depend on the H content and sp2/sp3 fraction in the film’s structure [32,36]. According to the cluster model, as proposed by Robertson and O'Reilly, the a-C:H films contain both sp2 and sp3 sites, with sp2 sites segregated into clusters embedded in a sp3 bonded matrix [37]. An increase of the hydrogen content into the DLC film’s structure causes to fracture the sp3 C-C bond, which are bonded clusters, and sp2 C=C aromatic which are inside of clusters. The fracture of sp3 C-C bonds and formation of sp3 C-H bond create a discontinuity in the DLC films network and consequently decrease the hardness and residual stress. In this research, by increasing the oxygen incorporation into the films’ structure, the H content of the film was decreased and the sp3 C-C bond content, which are bonded clusters were increased, although the sp2 (C=C) to sp3 (C-H and C-C) sites ratio were increased. It has been shown that by increasing the hydrogen content in the DLC films, sp3 C-C bond, hardness and residual stress were increased [36].

The oxygen doped DLC films were deposited by RF-PECVD method using the mixture of methane (CH4), argon (Ar) and oxygen (O2). By increasing the oxygen concentration in the precursor gas mixture up to 5.6 vol. %, the incorporation of oxygen into films’ structure was increased. This claim was confirmed by XPS, ERDA-RBS and ATR-FTIR spectroscopy results. Incorporation of oxygen into the films caused to change the films’ structures and consequently changed mechanical properties. By increasing the oxygen content into doped DLC films, the ratio of aromatic to olefinic C=C bonds was increased and the hydrogen content was reduced. This claim was also confirmed by Raman and ATR-FTIR spectroscopy results. Hardness and residual stress of the films were raised by increasing the oxygen content in the films due to an increase of sp3 C-C bonds between clusters and the decrease of C-H bond in the DLC films network.

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Fig. 1. The XPS spectra in the range from 0 to 1200 eV for un-doped DLC film and doped DLC film with 0.6 sccm oxygen flow rate.

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Fig. 2. The C1s peak and their sub-peaks (sp3,sp2,CO) for all films, (a) 0.0 sccm (b) 0.2 sccm (c) 0.4 sccm (d) 0.6 sccm (e) 0.8 sccm (f) 1.0 sccm flow rate oxygen. 11

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Fig. 3. The Raman spectra for DLC films deposited at different oxygen flow rate.

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Fig. 4. The ID/IG ratio as a function of the sp2/sp3 ratio.

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Fig. 5. The normalized absorption spectra which were derived from ATR-FTIR in the range from 2800 to 3000 cm-1 for (a) un-doped DLC film and doped DLC film with (b) 0.2 sccm (c) 0.4 sccm (d) 0.6 sccm (e) 0.8 sccm (f) 1.0 sccm oxygen flow rate.

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Fig. 6. The normalized absorption spectra which were derived from ATR-FTIR in the range from 1500 to 1800 cm-1 for (a) un-doped DLC film and doped DLC film with (b) 0.2 sccm (c) 0.4 sccm (d) 0.6 sccm (e) 0.8 sccm (f) 1.0 sccm oxygen flow rate.

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89.66 88.07 87.24 83.24 87.78 87.09

1.46 1.52 1.66 1.76 1.53 1.66

283.84 283.91 283.63 283.61 283.84 283.69

62.96 59.55 53.7 40.34 55.76 55.55

284.3 284.3 284.3 284.3 284.3 284.3

O at%

sp2/sp3 ratio

Raman parameters G-peak G-peak position width -1 (cm ) (cm-1)

0.43 0.48 0.61 1.04 0.57 0.56

1518 1528 1550 1576 1531 1545

10.34 11.93 12.76 16.76 12.22 12.91

1.28 1.35 1.15 1.3 1.27 1.36

PT

284.14 284.07 284.04 283.95 284.12 283.95

26.9 28.55 32.72 41.84 31.74 31.34

RI

C at%

Position FWH (ev) M (ev)

SC

C1s peak Position FWHM (ev) (ev)

Area (%)

NU

FWH M (ev) 2.78 2.81 3.1 2.71 2.95 3.03

sp3 peak

175 162 160 143 172 170

1.59 1.67 1.39 1.38 1.62 1.5

ID/IG ratio

0.30 0.53 0.71 0.94 0.39 0.47

AC CE P

TE

0 0.2 0.4 0.6 0.8 1

Position (ev) 285.43 285.54 285.72 285.43 285.64 285.54

sp2 peak Area Position FWHM (%) (ev) (ev)

MA

Oxygen concentration in feeding gas

CO peak Area (%) 10.14 11.9 13.57 17.82 12.49 13.11

D

Oxygen concentration in feeding gas 0 0.2 0.4 0.6 0.8 1

Table. 2. The chemical composition of un-doped DLC film and doped DLC film with 0.6 sccm oxygen flow rate that derived by the ERDA-RBS Designation film

un-doped DLC film doped DLC film with 0.6 sccm flow rate oxygen

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Element at% C H O 70 30 81 8 11

ACCEPTED MANUSCRIPT

Table. 3. The IR absorption peaks of the un-doped DLC film and doped DLC film with 0.6 sccm oxygen flow rate

RI

SC

A5=0.055 A6=0.035 A7=0.419 A8=0.070

NU

sp2 C=C aromatic sp2 C=C aromatic sp2 C=C olefinic C=O

MA

1550 1580 1650 1732

A’5=0.085 A’6=0.052 A’7=0.516 A’8=0.102

AC CE P

TE

D

5 6 7 8

Peak area of un- Peak area of doped DLC doped DLC film film with 0.6 sccm flow rate oxygen A1=0.017 A’1=0.019 A2=0.095 A’2=0.074 A3=0.009 A’3=0.008 A4=0.040 A’4=0.031

PT

Peak designation No. Wavenumb Configuration er (cm-1) 1 2960 sp3 CH3 2 2920 sp3 CH, CH2 3 2885 sp3 CH3 4 2855 sp3 CH2

Fig. 7. Hardness and residual of the DLC films as a function of different oxygen concentrations.

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ACCEPTED MANUSCRIPT

AC CE P

TE

D

MA

NU

SC

RI

PT

Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

Oxygen doping increased hardness of the DLC thin film.

AC CE P

TE

D

MA

NU

SC

RI

Oxygen doping increased residual stress of the DLC thin film

PT

Oxygen doped DLC thin film was deposited on silicon substrate by a PECVED method.

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